The present invention relates to a method and apparatus for applying a liquid coating mixture onto a continuously conveyed, elongated flexible support member (hereinafter referred to simply as a "web") in manufacturing a photographic film, a photographic paper, a material for a photomechanical process, a pressure-sensitive paper, a heat-sensitive recording paper, a magnetic recording tape, or the like. More particularly, the present invention relates to a method and apparatus for applying one or more types of coating solutions onto the web simultaneously using a slide hopper-type solution applying device.
When a coating solution is applied to a continuously conveyed web using a slide hopper-type solution applying device, a solution bridge (hereinafter referred to as a "bead") is produced between the leading end portion (hereinafter referred to as a "lip") of a solution injector and the web, and the coating solution is applied to the web by the bead. In applying the coating solution with the slide hopper device, the bead must be stably maintained. Hence, if the bead is subjected to external disturbances such as deformation of the web or contact of adhered materials, damage to the solution applying device, adherence of materials to the device, vibration of the device, pressure variations of an atmospheric gas or the like, then the film formed on the web may be affected. For example, streaking may result in the film extending in the conveyance direction of the web, thus reducing the quality and reliability of the film formed on the web.
The streaking may include streaks produced at the start of solution application, streaks produced when the film passes a jointed portion of two webs, or streaks produced by foreign matter adhered to the web passing the bead portion. Additionally, damage to the lip in direct contact with the bead or adherence of foreign matter to the lip due to drying and hardening or gelation of the coating solution may cause streaking. Particularly, streaks caused by the foreign matter adhering to the lip creates a serious problem in successively applying the coating solution to the web over a relatively long duration.
Various improvements have been proposed to prevent the above-mentioned streaking from occurring. For example, the following techniques have been proposed:
(1) As disclosed in U.S. Pat. No. 3,993,019, an acute portion of a lip is removed to make the lip obtuse to protect it against slight damage.
(2) As disclosed in Japanese Patent Un-examined Publication No. Hei. 1-206332, a lip is constructed to support the lower end portion of a bead.
(3) As disclosed in U.S. Pat. No. 4,292,349, to prevent adhesion of foreign matter due to the drying and hardening of a coating solution to be applied, a slide is constructed with a hood which minimizes evaporation of the coating solution.
However, the above-mentioned conventional techniques have been ineffective in preventing streaking from occurring over a relatively long duration. Particularly, when the coating solution has good wettability, for example, when the coating solution contains an organic solvent, the above-mentioned conventional preventive techniques are ineffective.
Specifically, in the above-mentioned technique (1), as shown in FIG. 5 when a lip 7 has an obtuse corner portion to hold the lower end portion of a bead 6, then the coating solution spreads wettingly beyond the lower end portion 1 of the lip 7, resulting in a streak being caused by the adhesion of foreign matter due to the evaporation and hardening of the coating solution.
Additionally, in above-mentioned technique (2), only providing the support means for holding the lower end portion of the bead 6 is insufficient in preventing evaporated and hardened material from forming due to spreading of the coating solution. Indeed, if the coating solution has good wettability, and if the lower end portion 1 of the lip for holding the lower end portion of the bead has an angle of more than 89.degree., then preventing adhesion of foreign matter due to the evaporation and hardening of the coating solution for a relatively long duration is impossible. Additionally, if the leading end portion of the lip is machined to have a complicated shape to hold the lower end portion of the bead 6, then the coating solution varies in quality in a solution collecting portion, and cleansing the solution collection portion is difficult. This results in reduced productivity of a coating film.
Further, a technique to restrict the evaporation of the coating solution to be applied, such as the above-described technique (3), is ineffective with respect to application of a composite solution which is hardened due to gelation. Also, with this technique, if the surrounding atmospheric gas has a pressure density due to the presence of a hood, then parts of the coating solution tend to condense forming droplets on the hood, which can then drop off the hood onto a film previously formed, causing damage to the film. Additionally, according to the film coating method using the slide hopper device, often the lower portion of the bead may have a pressure lower than that of the upper portion of the bead. Ordinarily, because the bead's lower portion is under a suction pressure, keeping the atmospheric gas pressure of the bead upper portion at a high level is difficult.